This is such a good idea. Messing with a website is absolutely something I'd have done instead of the typing practice I was supposed to be doing in my (small town public) middle school "Computers" class.
Yeah. My kid in Year 4 (Grade 3) has been doing this with his classmates. They fake their on-screen scores on TT Rockstars (a times tables game) and other shenanigans. So they might not be impressed if they have already discovered it.
There's all kinds of cool stuff you can do with devtools and inspecting a page though. If they've figured out how to change the text, start teaching them how styling works, or show them more advanced styling than they've discovered. Show them how to poke around in the console or see JSON payloads.